Pulsed Neutron Logging Ratio for CO2 Saturation Differentiation
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Solution Overview
Problem
Existing pulsed neutron logging techniques struggle to provide sufficient sensitivity and accuracy in differentiating carbon dioxide (CO2) saturation from other formation fluids such as oil, methane gas, and saltwater in reservoirs, particularly in carbon capture and sequestration (CCS) projects.
Innovation Solution
A method using a pulsed neutron tool with multiple detectors to analyze thermal capture decay data, decomposing it into formation and borehole contributions, and calculating a new ratio of capture counts from detectors at different distances from the neutron source to enhance sensitivity to hydrogen index, thereby improving CO2 detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional pulsed neutron logging techniques are used, then the measurement can be performed with standard equipment, but the sensitivity and accuracy in differentiating CO2 saturation from other formation fluids is insufficient
Solution Approach 1:
The patent segments the detection process by using multiple detectors at different distances from the neutron source. Each detector captures neutron capture events from different spatial zones, allowing the system to differentiate between formation and borehole contributions. This segmentation enables more precise CO2 saturation measurements by comparing signals from proximal and distal detectors, directly resolving the contradiction between measurement precision and device complexity.
Solution Approach 2:
The patent introduces a spatial dimension to the measurement by placing detectors at different axial distances from the neutron source along the tool axis. This dimensional approach creates a depth-of-penetration effect where proximal detectors measure near-field interactions and distal detectors measure far-field interactions, enabling differentiation of CO2 saturation with enhanced precision without excessive complexity.
2Measurement precision
If multiple detectors at different distances are used, then the sensitivity to hydrogen index and CO2 differentiation is significantly improved, but the device complexity and data processing requirements increase
Solution Approach 1:
The detection system is segmented into proximal and distal detector components, each serving a specific measurement function. The proximal detector captures signals from the immediate formation zone while the distal detector captures signals from deeper formation zones. This functional segmentation improves CO2 differentiation sensitivity while managing device complexity through modular detector design.
Solution Approach 2:
The multiple detectors serve multiple functions: they simultaneously measure hydrogen index, differentiate CO2 from other fluids, and provide depth-of-penetration information. This multi-functionality maximizes the utility of each detector component, improving measurement precision while justifying the increased device complexity through enhanced measurement capabilities.
3Device complexity
If traditional single-detector or close-spaced detector methods are used, then the device structure is simpler, but the ability to distinguish CO2 from saltwater and methane gas is limited
Solution Approach 1:
The patent adds the axial distance dimension to detector placement, creating a spatial gradient in measurement sensitivity. Detectors positioned at different distances from the neutron source experience different neutron flux distributions and capture different fluid signatures. This dimensional approach significantly improves fluid differentiation contrast between CO2, saltwater, and methane gas while maintaining manageable device complexity.
Solution Approach 2:
Different detectors are optimized for detecting specific fluid types based on their position. The proximal detector is more sensitive to fluids with higher hydrogen content, while the distal detector provides better contrast for CO2 detection. This local optimization of detector characteristics enhances overall fluid differentiation capability without requiring complex device architecture.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The new ratio provides significantly better sensitivity and contrast in distinguishing CO2 from other fluids, offering up to 8x better differentiation from saltwater and 3x better from methane gas compared to traditional methods, enhancing CO2 saturation measurements in reservoirs.
Implementation Method 1
One possible interaction is an elastic collision, also called elastic scattering, between a neutron n and a nucleus
Implementation Method 2
In an inelastic collision, also called inelastic scattering, a neutron collides with a nucleus, imparting a portion of the neutron's energy to the nucleus
Implementation Method 3
Epithermal and thermal neutrons can participate in a third type of interaction whereby the thermal neutron is 'captured' by the nucleus of an atom
Implementation Method 4
The energy that is transferred to the nucleus excites the nucleus, which subsequently emits a gamma (γ) photon when the nucleus relaxes
Data Source
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Figure 3
Figure 4~5
AI summary
Methods, tools, and systems for determining CO2 saturation in a porous formation using pulsed neutron logging are described. Embodiments of a pulsed neutron logging tool feature a pulsed neutron generator configured to emit pulsed neutrons into the formation and at least two detectors configured to receive emitted photons. The first detector is located closer to the neutron generator than the second detector. Embodiments of the method involve determining first detector formation capture counts indicative of neutron capture photons originating from the formation and detected at the first detector, determining second detector borehole capture counts indicative of neutron capture photons originating from the borehole and detected at the second detector, using the first detector formation capture counts and the second detector borehole capture counts to estimate the saturation of CO2 in the formation.